Steel sheets for automobiles are required to realize improvement of fuel efficiency and collision resistance. Accordingly, attempts are being made to increase strength of the steel sheets for automobiles. However, the improvement of strength is generally accompanied by a decrease of ductility such as press formability and accordingly makes it difficult to manufacture a component having a complicated shape. For example, in accordance with the decrease of ductility, a highly-worked portion fractures, or springback and wall warping are become significant to deteriorate dimensional accuracy. Therefore, it is not easy to manufacture a component by press-forming a high-strength steel sheet, especially, a steel sheet having 780 MPa tensile strength or more. Roll forming instead of the press forming can easily work a high-strength steel sheet, but its application is only limited to a component having a uniform cross section in a longitudinal direction.
Patent Literature 1 discloses a method called hot pressing having an object to obtain high formability in a high-strength steel sheet. By the hot pressing, it is possible to form a high-strength steel sheet highly accurately to obtain a hot-pressed steel sheet material having high strength.
Patent Literature 2 discloses a hot forming method having an object to obtain stable strength and toughness, and Patent Literature 3 discloses a steel sheet aiming at improved formability and hardenability. Patent Literature 4 discloses a steel sheet aiming at realizing both strength and formability, Patent Literature 5 discloses an art having an object to manufacture steel sheets having a plurality of strength levels from the same steel type, and Patent Literature 6 discloses a method of manufacturing a steel pipe having an object to improve formability and torsional fatigue resistance. Patent Literature 7 discloses an art to improve a cooling rate in hot forming. Non-patent Literature 1 discloses a relation between a cooling rate in quenching, and hardness and a structure of a hot-pressed steel material.
Incidentally, collision resistance of an automobile depends on not only tensile strength but also yield strength and toughness suitable for the tensile strength. For example, for a bumper reinforce, a center pillar, and the like, it is required that plastic deformation is suppressed as much as possible so as not to fracture soon even if they deform.
However, it is difficult to obtain excellent collision resistance by the aforesaid conventional arts.